It is generally desirable to increase as much as practical the amount of data which can be stored on an information storage disk. Data is generally stored on a disk along concentric circular tracks. Obviously, it is desirable to fit as many tracks as possible on a disk surface, which means that the tracks should be as narrow as practical and as close together as practical. However, it is also necessary to position a transducer over the middle of any desired track and to keep it there during rotation of the disk while data is written into the track or read from the track. As the tracks become ever smaller in width and are spaced closer and closer together, it becomes ever more difficult to correctly position the read and/or write transducer. In reality it is the ability to reliably position a transducer over the center of a desired track and to keep it so positioned during rotation of the disk which limits the practical density of the recording tracks. With magnetic storage disks, typically an open loop positioning system with no feedback control is used to coarse position either one or an array of read/write transducers over a desired track. With flexible magnetic storage disks, this usually is the only positioning system for the transducer. With rigid magnetic storage disks, coarse positionment is generally followed by fine positionment using a positioning system with feedback control to precisely position the transducer over the center of the desired track and to keep it there. The fine positioning system generally locks onto and tracks a magnetic pattern of some kind.
Most of these magnetic servo schemes use a portion of the recording surface for a magnetic servo pattern. This obviously reduces the amount of recording surface available for the recording of data. Accordingly, non-magnetic servo schemes have been proposed, which have the obvious advantage that the servo pattern will not reduce the amount of surface area available for magnetic storage of data.
U.S. Pat. No. 3,426,337 describes such a scheme in which an optical servo controls the transducer position. Unfortunately, in this scheme and in all such optical servo schemes known, a pattern of some kind must be recorded on or must be imprinted on the disk so that the servo control system has a reference. This requirement significantly increases the cost of achieving optical servo control of a magnetic disk transducer and has virtually foreclosed such servo control in the low cost flexible disk field.
It is an object of this invention to provide a servo control system for positioning a transducer with respect to an information storage disk without using a reference pattern on the storage disk.
Another object is to provide a practical optical servo system for control of the read/write transducer in a flexible magnetic disk storage system.